1. Field of the Invention
The invention is directed to a light-emitting diode with an active layer that emits photons and with a window layer that is transparent for the emitted photons and on which contacts for feeding current into the active layer are formed.
2. Description of the Related Art
Light-emitting diodes with the above-mentioned structure are known from U.S. Pat. No. 5,233,204. The known light-emitting diode comprises a light-generating region that is arranged on a light-absorbent substrate. A window layer is arranged above the light-generating region, the thickness of this window layer being selected such that light rays totally reflected at an upper side of the window layer are not directed to the substrate but rather are incident onto a lateral surface of the window layer and can emerge from the window layer at this surface. Additionally, the window layer has the job of distributing the current supplied into the window layer via a contact pad uniformly across the active region.
One disadvantage of the known light-emitting diode is that light generated in the active region immediately under the contact pad cannot escape without further effort. In particular, the light rays proceeding from the active region in the direction onto the contact pad are reflected directly back into the absorbent substrate by the contact pad.
European Patent Document EP 0 544 512 A discloses that the window layer be forgone and that interconnects structured in a tree-like manner be provided on the surface of the light-emitting diode instead. The specific quality provided by this approach is that the interconnects are insulated from the layer lying below up to the tips of the interconnect branches. The current is therefore only supplied in a punctiform fashion into the light-generating layers lying below. These locations, however, are uniformly distributed across the surface of the light-emitting diode. In order to increase the light yield, it is also proposed that beveled light exit surfaces be provided in the proximity of the light-emitting points so that the generated light can emerge from the light-emitting diode after a short distance. The beveled light exit surfaces are accomplished by a flat mesa-like fashioning of the layers lying under the interconnect. These layers are a matter of the doped cover layers required for the formation of a double hetero structure between which an undoped, light-emitting layer is arranged. Compared to the Prior Art, the light yield can be enhanced by a factor of 1.5 with such an arrangement.
Proceeding from this Prior Art, the object of the invention is to create a light-emitting diode with an enhanced light yield.
This object is inventively achieved in that the contacts are formed by interconnect lines and in that the window layer comprises lateral surfaces proceeding along the interconnect lines, the lateral surfaces being profiled with projections extending transversely relative to the interconnect lines.
By fashioning the contacts as interconnect lines, the current is injected into the active region not only in a punctiform fashion, but in a large-area manner. The photons generated in the active layer can leave the light-emitting diode via the lateral surfaces of the window layer. This also applies to those photons whose path proceeds in the window layer along the interconnect lines. Due to the profiling of the lateral surfaces with projections extending transversely relative to the interconnect lines, these photons also impinge a section of the lateral surface at an angle that is smaller than the angle for a total reflection. These photons are thus not lost but also contribute to the light yield. Experimentally, an increase of the light yield by a factor of 2 was found compared to the light-emitting diodes with a simple window layer known from the Prior Art.
In a preferred development of the invention, the projections are fashioned like a frustum of a pyramid.
Given a suitable angle of the frustum of the pyramid, a photon trajectory proceeding therein will exhibit a different incident angle at every incidence onto the sidewalls of the pyramidal frustum. It is therefore likely that, following multiple reflection, the incident angle will eventually become smaller than the critical angle for the total reflection, and the photon can emerge from the window layer.
Further advantageous developments include having the projections come to a point, having the projections comprise a triangular base area, having the projections form a sawtooth-shaped profile on the lateral surfaces, and/or having an apex angle of the projections coming to a point being less than 10xc2x0 in the point. An angle between a normal of sidewalls of the projections relative to a normal of the active layer may be provided which is between 45xc2x0 and 88xc2x0. The invention may include having the lateral surfaces extend across the active layer, and the projections provided may be prism-shaped. A profiled web edge having a constriction with a height to width ratio between 0.1 and 10 may be provided in an embodiment of the invention. Interconnect lines may be fashioned as interconnects running all around and along the edge of the light-emitting diode. These interconnect lines may be connected to a central contact pad by connecting track insulated from the window layer. These may also be provided on edge webs.